Small scale motors or actuators which are mainly utilized in domestic electric appliances, computers, automobiles, or other machineries are required to be produced with a miniatured scale, therefore light weight and high efficiency characteristics are obtained. Accordingly, magnet materials dominantly used for these devices are demanded to be fabricated with a small size, light weight and thin wall thickness. Moreover, in some applications, the magnets are required to be fabricated in more complicated geometries including providing uneven portions at the certain surface area thereof or providing through-holes.
As for the typical types of sintered permanent magnets, there are ferrite magnet, R--Co system sintered magnet, and R--Fe--B system sintered magnet (where R stands for rare-earth system), which the latter was proposed by the present inventors (Japan Patent Publication No. Sho 61-34242; U.S. Pat. No. 4,770,723; EP 0 101 552 B1).
Since rare-earth system magnets such as said R--Co system and R--Fe--B system magnets among the aforementioned magnets exhibit excellent magnetic characteristics compared with other types of magnets, so that they are preferably used in various applications.
Since the rare-earth system magnet, for instance R--Fe--B system sintered permanent magnet, has a maximum energy product ((BH)max) above 40 MGOe, and its maximum value exceeds 50 MGOe, therefore, exhibiting excellent magnetic properties. However, in order to realize such magnetic properties, alloy powders with certain compositions are needed to be pulverized into an average particle size of 1.about.10 .mu.m.
However, it should be recognized that when the particle size of alloy powders become smaller, the flowability of said pulverized powders will be deteriorated during molding. This will cause a scatter in the density of the molded products and reduction of the molding machine'life. Moreover, the dimension accuracy of the final sintered products will be scattered, resulting in that fabricating products with small scale and thin wall thickness will become more difficult.
Furthermore, since the rare-earth system magnets contain rare-earth system(s) and iron which are prone to be easily oxidized in an ambient atmosphere, the magnetic properties will be deteriorated due to oxidation, particularly when the particle size becomes smaller. This is more significant for R--Fe--B system sintered magnets, which possess excellent magnetic characteristics when compared to the conventional type of R--Co magnets, because certain type of compounds having a new structure produced by reaction of rare-earth system and B element are very active; said newly produced compound(s) is(are) believed to provide sources for magnetic characteristics. As a result, when the particle size of the alloy powders becomes smaller, the final sintered magnet had drawbacks of the deteriorated magnetic properties as a result of Oxidation.
Hence, for particularly improving the formability, several measures have been proposed; namely, addition of polyoxyethylene-alkylether or the like (Japan Patent Publication No. Hei 4-80961), addition of paraffin or stearic acid salts, besides the aforementioned ether (Japan Patent Publication No. Hei 4-80962, JPP No. Hei 5-53842), or addition of the olefine acid (JPP No. Sho 62-36365).
Although the formability was improved to some extent, it was found that there was a limitation of such improved formability, so that it is still difficult to fabricate products having small scale, thin wall thickness, or intricate shape.
Moreover, as alternative production methods for magnets with characteristic geometrical features including a thin wall thickness and a small scale by adding the aforementioned binder and lubricant for further improvement of the formability, there were additional inventions proposed; namely a production method by which a lubricant made of the myristic acid ethyl or oleic acid and the saturated aliphatic carboxylic acid or unsaturated aliphatic carboxylic acid, is added to the alloy powders prior to molding and kneading, granulated and molding (Japan Patent Application Laid-Open No. Sho 62-245604), and a production process by which the saturated aliphatic carboxylic acid or unsaturated carboxylic acid is added to the paraffin mixture, and molded after granulated and kneaded (JPALO No. Sho 63-237402).
Even with the aforementioned modification, it was found that the bonding strength among powder particles was not sufficiently high enough, and the granulated powder was easily broken, resulting in that a sufficient flowability was not achieved.
In order to enhance the formability or to improve the binding strength of powder particles, it can be done to increase the amount added of various types of binder or lubricant. However, if a large amount of these additives is applied, a residual oxygen as well as residual carbon in the sintered products will increase, due to the fact that the R component in the alloy powder of the rare-earth system and the binder will chemically react. This will cause the deterioration of the magnetic properties. Accordingly, there was a limitation for the amount of adding these additives.
Furthermore, although this is not for the rare-earth system alloy powders, an addition of 1.5.about.3.5 wt % of methylcellulose and a certain amount of glycerine and boric acid to the alloy powders of Co system superalloy was proposed (U.S. Pat. No. 4,118,480); these additives were used as a binder for the compression molding of the Co system superalloy powder. Moreover, as a binder for a tool steel alloy powder for the injection molding technique, the additive composed of 0.5.about.2.5 wt % of methyl-cellulose, water, plasticizer such as glycerine, lubricant such as wax-emulsion, and mold-separator was proposed (Japan Patent Application Laid-Open No. Sho 62-37302).
However, the added amount of the aforementioned binder additives is relatively larger than 0.5 wt % in order to maintain a certain level of flowability as well as mold strength. Furthermore, a simultaneous addition of various types of binders such as glycerine with methylcellulose is indispensable, so that a remarkable amount of residual oxygen and carbon can be found even after the injection molding, compression molding, degreasing process, or sintering process. As a result, the residual oxygen and carbon showed an adverse effect on magnetic properties, in particularly the rare-earth system magnets; so that these additives can not be easily applied.
Furthermore, a process is known to add 0.6.about.1.0 wt % of polyvinylalcohol as a binder to powder having an average particle size of less than 1 .mu.m for the oxide powders including ferrite or the like, followed by producing granulated powders by the spray-dryer equipment, molding, and sintering.
However, in either aforementioned methods to be used, a large amount of binder with more than 0.6 wt % is added to oxide powders, so that a remarkable amount of residual oxygen and carbon can be found in the sintered products even after the degreasing. Therefore, the aforementioned methods being proposed for the oxide powders cannot be applied to the rare-earth system system alloy powder in the present invention, because said rare-earth system alloy powder contains substances which are very sensitive for oxidation and carbonization; hence once these components are oxidized and/or carbonized, the original magnetic properties will be extensively deteriorated.
Particularly, even if a large amount of binder is used for the oxide powders, a certain amount of residual carbon can be controlled by the degreasing process and the subsequent sintering process in air through which some of residual carbon can be burned out. On the other hand, since the magnetic properties of the rare-earth system alloy powders of the present invention will be adversely influenced by oxidation, degreasing and sintering processes cannot be conducted in air. Hence, a large addition of the amount of binder will have very bad influences on the magnetic properties of the final sintered products.
As discussed in the above, several improvements were proposed to add various binder or lubricant to alloy powders prior to the sintering process, or to improve the formability by the granulation method. Unfortunately, it is difficult to fabricate the rare-earth system magnets having excellent magnetic properties and unique configurations with small scale, thin wall thickness, and/or intricate shape, as currently demanded from various sectors in the technology, through any one of the above mentioned proposed ideas.